Autocrypt is a set of guidelines for convenient end-to-end-encryption of e-mails. Developers are working on bringing it to several different e-mail programs.
Delta Chat is a modern messenger. It is like email in a new dress. Just better, safer and user-optimised.
Enigmail is a seamlessly integrated security add-on for Mozilla Thunderbird. It allows you to use OpenPGP to encrypt and digitally sign your emails and to decrypt and verify messages you receive.
K-9 Mail is an open source email client focused on making it easy to chew through large volumes of email
muacrypt provides a command line tool and a Python API to help mail agents, both user and server-side, integrate and manage automated e-mail end-to-end encryption with https://autocrypt.org.
Autocrypt Level 1 implemention using PGPy . Initially part of py-autocrypt, which uses GnuPG.